Supplemental restraint systems or airbags have quickly become an important safety feature in automobiles today. Current airbag deployment technology uses controlled combustion or an "explosion" to rapidly deploy the airbag. The chemicals that create this controlled explosion are housed in a chamber or housing.
In order to assure proper airbag deployment, the chemicals and the subsequent combustion must be contained within the chamber in such a manner as to assure complete, controlled and predictable combustion within certain given parameters. One requirement to meet such a set of given parameters is the structural integrity of the combustion chamber. That is, the chamber must be configured and fabricated such that it maintains its integrity through the combustion process and subsequent airbag deployment.
One known chamber configuration includes a substantially tubular member having an open end and an opposing, partially occluded end. The chamber expands in a step-wise manner from the partially occluded end to the fully open end. The open end of the chamber is that portion of the chamber that is at the largest diameter of the chamber, and is subsequently mounted to another component of the airbag assembly.
In a typical manufacturing process for the chamber, the chamber is formed from common carbon steel materials, such as American Iron and Steel Institute (AISI) 1006 to 1008. The steel is cold drawn to form the various requisite step-wise expansions in the housing body.
It has been observed that in order to assure structural integrity of the chamber, the thickness of the steel must be substantial, about 0.062 gauge. It has also been observed that at relatively low temperatures during impact testing, the chamber can split or fail at particular points along the chamber body. Specifically, the chambers have been observed to fail from an edge of the open end, longitudinally along the chamber body.
Microscopic examination of the failed samples has shown that the grain structure of the steel elongates in the direction of drawing, and that cracks in the chamber tend to propagate along the elongated grain boundaries. As a result of the weakened grain structure, the cracks tend to grow substantially longitudinally along the chamber body.
It is believed that failure of the combustion chambers during testing is due, in part, to stress cracks that are induced in the edge of the material as a result of drawing. Specifically, cracks develop at the edge and propagate, along the elongated grain boundaries, in the direction in which the steel is drawn.
One method to alleviate the cracking problem that has shown some success is to machine the edge of the chamber housing body to remove the cracks. Although this method has been somewhat successful in reducing cracking in the housing, it requires considerable time and is a labor-intensive and thus costly effort.
Accordingly, there continues to be a need for a combustion chamber housing having a high degree of structural integrity which chamber is formed from relatively common carbon steel materials. Further, there continues to be a need for a method for forming such housings, which method uses efficient and cost-effective parts and processes for manufacturing the housing.